1. Field of the Invention
The present invention relates to an electro-optical device and a method of driving the same, and, in particular, to the method of gradation image display for obtaining the expression having neutral color tone or intermediate brightness, by utilizing a thin film transistor (hereinafter referred to as TFT) as a switching device for driving. The present invention relates, in particular, to the complete digital gradation display for performing a gradation display without applying any external analog signal to an active device.
2. Description of the Prior Art
A liquid crystal composition can easily be oriented in a parallel direction or in a vertical direction to an external electric field existing outside thereof, because the dielectric constant of the liquid crystal composition in a direction parallel to the molecule axis thereof is different from that in a direction vertical to the molecule axis. The ON/OFF display, i.e. the display in a degree of brightness, is carried out by taking advantage of the anisotropy in dielectric constant, and whereby controlling the amount of transmitted light or the degree of light dispersion. As a liquid crystal material, TN (twisted nematic) liquid crystal, STN (super-twisted nematic) liquid crystal, ferroelectric liquid crystal, antiferroelectric liquid crystal, polymer liquid crystal or dispersion liquid crystal are conventionally known. It is known that it takes a certain period of time before a liquid crystal responds to an external voltage, rather than an infinitely short period of time. The value of the response time is proper to each liquid crystal material: in case of TN liquid crystal, it is several 10 msec, while in case of STN liquid crystal, it is several 100 msec, and in case of ferroelectric liquid crystal, it is several 10 microsec, while in case of dispersion or polymer liquid crystal, it is several 10 msec.
Of the electro-optical device utilizing liquid crystal, a method of obtaining the most excellent image quality is the one taking advantage of an active matrix method. In case of a conventional active matrix type liquid crystal electro-optical device, a thin film transistor (TFT) was used as an active device, while amorphous or polycrystalline semiconductor was used for TFT, and either P-type or N-type TFT is utilized for one picture element. Namely, an N-channel TFT (also referred to as NTFT) is generally connected to a picture element in series. The NTFTs are provided at the intersections of the signal lines arranged in a matrix form. The ON/OFF of a liquid crystal picture element is controlled by taking advantage of the fact that a TFT is turned in an ON state when signals are applied to the TFT through the two signal lines connected thereto. By thus controlling the picture element, a liquid crystal electro-optical device of large contrast can be achieved.
In case of the active matrix method as mentioned above, however, gradation display of brightness or color tone was very hard to carry out. Actually, a method utilizing the fact that the light transmission of liquid crystal is varied dependent upon the level of voltage applied thereto, was under examination. This meant, for example, that a proper level of voltage was supplied between the source and the drain of the TFT in a matrix, from a peripheral circuit, and that the same level of voltage was applied to a liquid crystal picture element by applying a signal voltage to a gate electrode under the condition.
In case of the abovementioned method, however, the voltage actually applied to the liquid crystal differed by at least several % in individual picture elements, owing to inhomogeneity of the TFT or to the inhomogeneity of a matrix wiring. On the other hand, the voltage dependency of light transmission of a liquid crystal has an extremely strong non-linear characteristic, and light transmission would drastically differs for the difference even by several %, for the light transmission changes drastically at a certain voltage. For this reason, a 16 gradation was practically an upper limit.
For example, a voltage range of so-called transition region, that is a voltage range in which light transmission varies from ON state to OFF state, is 1.2 V in case of TN liquid crystal material. Hence, in order to achieve 16 gradations, a minute control of 75 mV voltages which are quotients of 1.2 V divided by 16 is necessary and production yield has been extremely low.
The difficulty in carrying out gradation display was an enormous drawback of a liquid crystal display device in terms of competitiveness with a CRT (cathode-ray tube) as a conventional and general display device.
An object of the present invention is to propose a novel display device having a thin film transistor formed over a substrate. According to the present invention, an insulating film including an inorganic material is provided over the thin film transistor, and a leveling film including an organic resin is formed over the substrate and covers the thin film transistor. Further, a pixel electrode is formed on the leveling film and is connected to a semiconductor film of the thin film transistor through an opening provided in the leveling film, in which an edge of the leveling film at a periphery of the opening is rounded, and a surface of the pixel electrode is rounded along the rounded edge of the leveling film.